Update docker/build-push-action action to v6.15.0 (#473)

| datasource  | package                  | from    | to      |
| ----------- | ------------------------ | ------- | ------- |
| github-tags | docker/build-push-action | v6.13.0 | v6.15.0 |

Signed-off-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>
Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

.github/workflows/ci.yml

@@ -72,7 +72,7 @@ jobs:
         uses: docker/setup-buildx-action@6524bf65af31da8d45b59e8c27de4bd072b392f5 # v3.8.0
 
       - name: Build Plus Docker Image
-        uses: docker/build-push-action@ca877d9245402d1537745e0e356eab47c3520991 # v6.13.0
+        uses: docker/build-push-action@471d1dc4e07e5cdedd4c2171150001c434f0b7a4 # v6.15.0
         with:
           file: docker/Dockerfile
           tags: nginx-plus